498 NICHOLS— COLOR-PHOTOGRAPHS OF 



violet. (Photographs were shown at the meeting illustrating these 

 and similar cases.) 



The work of Lenard and others using a very different pro- 

 cedure has shown the spectra of these substances to consist oi broad 

 overlapping bands distinguishable from each other chiefly by the 

 mode of excitation, duration, and the influence of temperature and it 

 is clear that in these and similar cases we have to do with two or 

 more such bands. 



In the first of these two examples the change of color may obvi- 

 ously be explained by the more rapid decay of the band of shorter 

 wave-length ; an hypothesis readily verified by observing the sub- 

 stance as seen through the disk with a spectroscope. What appears 

 as a single very broad band collapses promptly from the green end 

 when the exciting circuit is broken, leaving the red portion only, 

 which persists for many seconds. 



We have to do then with a combination of a band of very 

 short duration (having its crest in the green) and a band of long 

 duration in the red and the color at any time after the close of 

 excitation is the sum of the instantaneous values of the two com- 

 ponents. 



Since the decay of the green band is very rapid indeed, its in- 

 tensity becoming negligible in a small fraction of a second, the 

 appearance of the phosphorescent surface when observed by ordinary 

 methods is chiefly due to the red component and it appears of a 

 much more ruddy color than when viewed through the rapidly re- 

 volving disk. The same is true of all the other barium sulphides 

 studied. 



The Ca and Sr sulphides in general appear as greenish blue, pure 

 blue or even violet as in the example cited (No. 3) when viewed 

 in the ordinary manner ; but in the phosphoroscope they are green, 

 going over into blue or violet after the cessation of the periodic 

 excitation. Here obviously we again have to do with two bands of 

 which the green is of very short duration, while it is not in this 

 case the band of longer wave-length which persists but that of 

 greater refrangibility. It appears in general that these sulphides fall 

 into two distinct classes : Those of which the persistent band is of 

 longer wave-length, i. e., red, chiefly if not exclusively barium 



